(1) Field of the Invention
The present invention relates to an apparatus for detecting an external force applied to, for example, a robot hand arranged on the top end of an arm of a robot.
(2) Description of the Related Art
Recently, there is a wide-spread introduction of industrial robots into production lines. However, since a position control type robot operates regardless of any restricting force caused by environmental factors, it is very difficult to use this type of robot for the automation of an assembly line in which a delicate force adjustment is required. This is because such an assembly operation involves many operations in which the high precision measurement of the relative positional relationship is more important than the absolute positions of parts, such as the fitting and attaching of parts.
If the fitting of a part is carried out by using a position control type robot, it is necessary to precisely program the robot coordinates for the moving locus determined from the object of assembling, and to precisely control the top end according to the predetermined locus. However, in practice with a position control type robot, it is very difficult to increase the precision of the absolute position because of inherent mechanical errors in the robot or because of control errors. Accordingly, in a robot used in such an assembly operation, a compliance mechanism is arranged on a wrist between the robot and a hand, to absorb any error in the relative position between the robot and the object.
However, since only a small quantity of error can be amended or absorbed, i.e., corrected, by the compliance mechanism arranged on the wrist, if the error of the relative position between the robot and the object is large, this error cannot be completely amended or absorbed. Accordingly, in the conventional robot of this type, a force-detecting apparatus (force sensor) is positioned so that it detects the above-mentioned error of the relative position as a reaction force acting on the wrist and corrects the position by feedback of the detected reaction force to the robot, so that the reaction force becomes zero. In short, control of the robot is changed from position control to force control.
In this arrangement, preferably the force sensor arranged on the wrist is rigid; a compliance mechanism is not preferred for the force sensor. This is because that if the wrist is composed of the compliance mechanism, a robot hand holding an article easily vibrates due to an outer force such as an inertia force applied to the article at the time the arm stops. Therefore, a robot having, for example, four degrees of freedom cannot cancel the vibration in the directions of six degrees of freedom, which degrades the reliability of feed-back control of the robot by using the sensor. Also, a sophisticated technique is required in an assembling operation these days, and the operation is conducted in a complicated manner. In such a situation, if the robot wrist is too soft, the robot cannot bear the weight of the article or the hand itself and becomes incapable of the assembling operation.
In the conventional force-detecting apparatus (as explained in detail later), since the output of one strain gauge for detecting force or torque includes a plurality of force components, the various components of the force vectors cannot be independently detected. Accordingly, a complicated matrix operation circuit must be constructed to obtain the force components in the respective directions and the torque-separated outputs based on the outputs of the respective strain gauges, and furthermore, complicated operation expressions must be introduced for analyzing and calculating the force components and torques of the respective directions based on the outputs of the respective strain gauges, and thus a large amount of time is required for the operation. Accordingly, if this force-detecting apparatus is used for driving and controlling a robot, the control circuit becomes complicated and the cost is increased. Furthermore, a large amount of time is necessary for conversion and separation of the respective components of the external force and it is impossible to immediately control the robot at a high speed in correspondence to the external force.